@noobian
Please disable the internal updater for Jackett ("Disable auto update" checkbox). It is a known, still not resolved external bug, occurring in all cases where Jackett is started via systemd unit, so not related to DietPi.
Ref: https://github.com/MichaIng/DietPi/issues/2593

But Sonarr and Radarr should not have such an issue. What you mean by "they were gone"? Did you check service status and logs before reinstalling them?

Sry, I did not check service-status nor logs, just did a restart, and Sonarr and Radarr both could not be reached through their typical webaddress ... after reinstall they worked fine.
Would you mind, posting me the two commands (service status, and get logs) here, so I will make sure to first call those, before reinstalling, in case this happens again?